The use of copper or various copper alloys as lead in wires in incandescent lamps has been in commercial practice for many years. More recently, dispersion strengthened copper has been used for such lead wires. (See McMillan U.S. Pat. No. 4,138,623). This dispersion strengthened copper wire was normally a "Glidcop" AL-20 wire or equivalent containing 0.20% aluminum oxide calculated as the metal equivalent and which had a thin copper (unstrengthened) sheath surrounding an inner core of the internally oxidized dispersion strengthened copper. This material, particularly when nickel plated to reduce release of contaminants from the underlying copper sheath, enabled lamp manufacturers to eliminate tie wires formerly used to support the plain copper or copper alloy leads and thus reduce cost.
Graves et al in U.S. Pat. No. 4,208,603 found that to improve the resistance to embrittlement of the tungsten filament due to nickel plated on copper sheathed dispersion strengthened copper, removal of the copper sheath, or production of the wire without the copper sheath, enabled plating of nickel directly onto the bare dispersion strengthened copper wire and gave a superior bonding of the nickel deposit onto the copper alloy surface. This structure was found to be less prone to nickel migration during lamp operation. Nickel migration was believed responsible for early filament failure due to embrittlement. As shown in U.S. Pat. No. 4,208,603, removal or omission of the copper sheath followed by nickel plating resulted in an improvement in filament life.
A problem still remained, however, with attack on the tungsten filament as a result of residual hydrogen on the tungsten and residual free oxygen in the lead wire or within the environment of the bulb albeit in very low concentrations. These elements combine to form water. Water vapor present in the system reacts with tungsten under incandescent temperature conditions to form tungsten oxide and hydrogen ions. The oxide vaporizes and condenses on the inner surface of the envelope. Ambient hydrogen ions under the condition of the lower temperature of the envelope reduces the tungsten to the metal and regenerates water vapor thereby completing what is known in the art as the "water cycle". To control plating out of tungsten onto the glass envelope, it has become the practice to include a "getter" at the outer end of the lead wire in the lamp reactive with the oxygen to reduce the deleterious effect thereof, and interrupt the "water cycle". Zirconium metal is a "getter" material which can be used for this purpose. Obviously, however, the inclusion of a "getter" in a lamp increases cost not only in respect of the cost of the material, but also in the added operation of applying it to the lead wire.
In the following discussion dispersion strengthened copper may be referred to as "DSC". Reference to an aluminum metal content of a given percentage will be understood as an equivalent amount of aluminum metal albeit present as the refractory oxide, aluminum oxide.
Thus, the art has progressed to the point where it was using dispersion strengthened copper (DSC) wire leads, without tie wires. Because undeoxidized DSC containing refractory metal oxide to any extent contains residual free oxygen and is thus a source contributing to the "water cycle", control of contaminents from the wire was achieved by nickel plating with improved bond, or by use of any auxiliary getter, or a combination of both.
Also, as known to those skilled in the art (See U.S. Pat. No. 4,138,623; Col. 2., Lines 2-14) lead wires are composed of three electrically conductive segments; an outer conductor and an inner conductor connected together through a short intervening segment of dumet wire. This segment is adapted and dimensioned to traverse the steam press portion of the glass stem to provide a seal between the inside and outside of the glass envelope. Dumet, which is a 40-43% nickel balance iron alloy, because of its favorable thermal expansion characteristics reduces the stresses due to differences in thermal radial expansion of the wire and the glass in the formation of the stem press and in actual use. The heat of softening the glass to make the stem press seal (about 1200.degree. F.) is sufficient to soften copper metal or copper alloys in a normal lead wire. A DSC wire is better able to withstand such temperature. It has now been found that use of nickel plating and/or use of an auxiliary use of nickel plating and/or use of an auxiliary "getter" and the attendant costs of each can be avoided while still maintaining the desired objective of eliminating the need for tie wires. This is achieved by use of a deoxidized internally oxidized dispersion strengthened copper wire, and particularly a deoxidized internally oxidized dispersion strengthened copper wire having a lower aluminum oxide (or equivalent refractory oxide) content than heretofore used. A principal source of free oxygen in the lamp environment is thereby substantially removed and the "water cycle", thus effectively controlled. Use of the lower oxide deoxidized DSC also presents advantages of greater ductility which facilitates fabrication and provides better electrical conductivity. A "getter" for any extraneous free oxygen can be incorporated in the body of the wire rather than topically applied after fabrication.
Still further, when the refractory oxide content in the DSC is as low as 0.15% aluminum equivalent, it has been found that use of a dumet segment can be avoided, if desired.